1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus, such as a copying machine, a printer or a facsimile, which creates an adjustment sheet used for concentration adjustment (xcex3-correction) of reference colors which constitute a color printing output image.
2. Description of the Related Art
In an image forming apparatus, such as a color printer, a color concentration of printing may shift from a reference value due to change with passage of time, and, thus, it is required to make a color correction. Conventional image forming apparatuses adopt various color correction methods.
For example, there is a method of determining an amount of correction of concentration by comparing a color sample sheet enclosed in a package of a printer with a print of a test pattern, which is stored in the printer. Moreover, there is a method of correcting a concentration curve by reading concentration of a figure printed on a print paper by a scanner or a concentration measurement apparatus and calculating a difference between a color of the drawn figure and an actual color. Furthermore, there is a method of correcting a concentration curve by selecting a concentration, which is equal to a reference concentration printed on a previously prepared template, the selection being made by printing all selectable concentrations on a print paper by a printer of which concentration curve is to be corrected.
A concentration value of print data sent from a host apparatus to an image forming apparatuses such as a printer is normally specified by one of numerical values of 0 to 255 expressed by 8 bits per one color. A process called xcex3-conversion is performed so that the concentration in a printing result matches a desired concentration with respect to each concentration value. The xcex3-conversion is a process that obtains, when a concentration is expressed by 8 bits, output values 0-255 with respect to input values 0-255 using a conversion table stored in a memory of a controller.
In a graph having horizontal and vertical axes of the same scale, if an input value is assigned to the horizontal axis and an output value is assigned to the vertical axis, the input value becomes equal to the output value and the xcex3-conversion process is unnecessary when the input-and-output characteristic of a printer engine is expressed by a straight line having an inclination of 45 degrees. However, the input-and-output characteristic of a printer engine does not have linearity, and the xcex3-conversion process is indispensable so as to obtain a proper image by the printer.
Moreover, even if printer engines are of the same model, generally the input-and-output characteristic varies according to individual differences. The input-and-output characteristic varies also with passage of time and further the environment where the printer is placed. Therefore, it is necessary to rewrite the contents of the conversion table in accordance with a change in the input-and-output characteristic.
If the contents of xcex3-conversion table are expressed by a graph in which an input value and an output value are assigned to the horizontal axis and the vertical axis, respectively, the xcex3-conversion table is indicated as a curve. For this reason, the process to rewrite the contents of xcex3-conversion table may be referred to as a xe2x80x9cgamma curve correctionxe2x80x9d. Moreover, the contents of xcex3-conversion table after correction may be referred to as xe2x80x9ccorrected xcex3xe2x80x9d or xe2x80x9ccorrected xcex3-curvexe2x80x9d.
FIG. 1 is an illustration showing a structure of a conventional printer system. The printer system shown in FIG. 1 comprises a host computer 1 and a printer engine 2. A printer controller 3 is provided in the printer engine 2. The data to be printed is specified by the host computer 1. The printer controller 3 creates image data according to instructions of the host computer 1 so as to perform a printing operation. After being developed on the frame memory according to the specified concentration, the image data to be printed is sent to the printer engine 2, and the printing operation is completed.
As shown in FIG. 2, the printer engine 2 comprises a central processing unit (CPU) 31, a random access memory (RAM) 32, a program read only memory (ROM) 33, a font ROM 34, a host interface (I/F) 35 and an engine I/F 36, and these parts are mutually connected through a bus. The CPU 31 controls the entire printer controller 3 based on programs stored in the program ROM 33, mode instructions from a panel apparatus and commands from the host computer 1. As mentioned above, a control program of the printer controller 3 is stored in the program ROM 33, and pattern data of a font etc. is stored in the font ROM 34.
The RAM 32 functions as a work memory of the CPU 31, and is used as an input buffer of input data, a page buffer of print data, a memory for download fonts, etc. The engine I/F 36 performs communication with the printer engine 2 so as to exchange commands and status and print data. The host I/F 35 is an interface which communicates with the host computer 1, and is usually Centronics I/F or RS232C I/F. The printer engine 2 may be connected with the host computer 1 through networks such as Ether Net or Local Talk.
Here, a concentration of an image printed by the printer engine may differ from a concentration which the printer controller intends to print due to a change in the surrounding environment or a change with passage of time. In such as case, in the printer controller provided with a reading apparatus such as a scanner, the printed paper output from the printer engine is read by the scanner. Then, the concentration value (output result), which is the result of reading, is compared with the concentration value (expected output value) which the printer controller intends to print. If there is a difference, the concentration value used for drawing is corrected (the above-mentioned xcex3-correction) so as to absorb the difference. Then, a printed matter can be obtained which the printer controller intends to, that is, which the host computer intends to.
In an image forming apparatus which is not provided with a reading apparatus such as a scanner, the same effect can be obtained by inputting the input data to the printer controller 3 by determining, as shown in FIG. 3, whether the printed result output from the printer engine 2 has a higher concentration or a lower concentration by a visual check of a human 4 without using a reading apparatus such as a scanner. In this case, since it is difficult to determine the concentration by human eyes unlike reading apparatuses such as a scanner, a sheet referred to as an adjustment sheet 5 is used so as to help the visual check so that the determination can easily made even by the human eyes.
In the adjustment sheet 5, when an adjustment is performed with respect a particular concentration, reference concentration patterns (B of FIG. 4) used as a reference and test concentration patterns (A of FIG. 4) which indicates concentrations near the particular concentration including the particular concentration are arranged in an alternative sequence. By comparing the concentration patterns and selecting a number at which the concentration of the test concentration pattern matches the concentration of the reference concentration pattern, the xcex3-correction value is obtained based on the concentration value (output result) indicated by the number attached to the selected test concentration pattern and the concentration values (expected output value) which the controller intends to represent. Thus, a desired concentration can be obtained by reflecting the thus-obtained xcex3-correction in the printing process. It should be noted that an example of such a process is disclosed in Japanese Patent Application No. 11-314395 filed by the present inventor. As for the reference concentration pattern, a pattern, which is hardly influenced by a change with passage of time or an individual difference, is used. For example, what is necessary to make 50% of concentration of the highest concentration of the printer engine 2 is just to use a checkered pattern as shown in FIG. 5. However, if each dot constituting the pattern corresponds to a single pixel (dot) of the printer engine 2, 50% concentration cannot be made stably since the concentration is influenced by the output characteristic of the printer engine 2. Therefore, the pattern, which is not influenced by the output characteristic, can be made by using the checkered pattern (FIG. 6) in which a constituting point consists of several dots. By performing the above-mentioned process, an adjustment of output characteristic, that is, an adjustment of concentration can be achieved even by a printer controller having no image reading apparatus.
In a case of a color printer, a concentration adjustment must be performed on colors other than black (hereafter may be referred to as xe2x80x9cKxe2x80x9d), such as cyan (may be referred to as xe2x80x9cCxe2x80x9d), magenta (may be referred to as xe2x80x9cMxe2x80x9d) and yellow (may be referred to as xe2x80x9cYxe2x80x9d). With respect to magenta and black, the concentration adjustment method using the above-mentioned adjustment sheet shown in FIG. 4 is performed for each individual color. With respect to cyan and yellow, a color mixture gray patch (refer to a section E of FIG. 7), which consists of cyan, magenta and yellow, is output by printing, and comparison is made with the black background (refer to a section D of FIG. 7) which represents the reference concentration. Accordingly, the concentration adjustment is performed on all four colors. However, in many cases, color tone appears in the gray (CMY mixture gray) created by blending C, M and Y, which gives a user a delicately different impression when comparing with a monochrome gray representing the reference concentration. Therefore, it is difficult to select a patch having the same concentration with the reference concentration, and variation may be made by each person who performs the test.
On the other hand, the printer controller has drawing modes such as resolution or character/photograph, and if the mode is different, the concentration of output from the printer engine may also differ. For example, a phenomenon either dark or light happens in the result of output according to a resolution of 600 dpi and the result of output according to a resolution of 1200 dpi. Moreover, there is a case in which a plurality of printer engines are assigned to a printer controller. In such a case, it is natural that the result of output differs. Further, also in the same printer engine, the result of output changes according to an amount of toner used or a service time of consumable parts. Furthermore, in the case of a color printer, the concentration characteristics differ also among composition reference colors (cyan, magenta, yellow, black, etc.). The amounts of C, M and Y blended vary due to the difference in output, and it is considered that a CMYK gray patch does not serve as a reference.
It is a general object of the present invention to provide an image forming apparatus in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an image forming apparatus which lessens the sense of incongruity between CMY mixture gray and K monochrome gray so as to achieve an easy color adjustment.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an image forming apparatus comprising: concentration pattern data producing means for producing reference concentration pattern data and adjustment concentration pattern data, the reference concentration pattern data representing a predetermined reference concentration and the adjustment concentration pattern data representing a plurality of concentrations within a predetermined concentration range including the predetermined reference concentration; printing means for printing an adjustment sheet based on the reference pattern data and the adjustment concentration pattern data so that a reference pattern and a plurality of test patterns are printed on the adjustment sheet, the reference pattern indicating the predetermined reference concentration and the test patterns indicating the concentrations indicated by the adjustment concentration pattern data; and concentration adjusting means for adjusting concentration data of image data to be printed in accordance with an adjustment concentration value detected by comparing a concentration of the reference pattern with concentrations of the test patterns printed on the adjustment sheet, wherein the concentration pattern data producing means produces the reference concentration pattern data and the adjustment concentration pattern data so that one of a color of the reference pattern and a color of the test patterns is added to the other of the color of the test pattern and the color of the test patterns.
In one embodiment of the present invention, the color of the reference pattern may be gray consisting of monochromatic black, and the color of the test patterns may be gray consisting of a mixture of cyan, magenta and yellow, and wherein the concentration pattern data producing means produces the reference concentration pattern data and the adjustment concentration pattern data so as to add the color of the reference pattern to the color of the test patterns so that the printing means prints the reference pattern in gray consisting of monochromatic black and prints the test patterns in gray which is a mixture of cyan, magenta yellow and black.
In another embodiment of the present invention, the color of the reference pattern may be gray consisting of monochromatic black, and the color of the test patterns may be gray consisting of a mixture of cyan, magenta and yellow, and wherein the concentration pattern data producing means produces the reference concentration pattern data and the adjustment concentration pattern data so as to add the color of the test patterns to the color of the reference patterns so that the printing means prints the reference pattern in gray consisting of monochromatic black and a mixture of cyan, magenta and yellow, and prints the test patterns in gray consisting of cyan, magenta and yellow.
Additionally, there is provided according to another aspect of the present invention a method of adjusting a concentration of gray in a color image printed by an image forming apparatus, comprising the steps of: preparing a reference pattern and a plurality of test concentration patterns by the image forming apparatus, the reference pattern indicating a reference concentration of gray and the test patterns indicating concentrations of gray within a predetermined concentration range including the reference concentration, the gray of the test patterns being a mixture of cyan, magenta, yellow and black, the gray of the reference pattern being monochromatic black; visually comparing the test patterns with the reference pattern so as to select one of the test patterns of which concentration matches the concentration of the reference pattern; and adjusting the concentration of gray in accordance with the concentration of the selected one of test patterns.
Further, there is provided according to another aspect of the present invention a method of adjusting a concentration of gray in a color image printed by an image forming apparatus, comprising the steps of: preparing a reference pattern and a plurality of test concentration patterns by the image forming apparatus, the reference pattern indicating a reference concentration of gray and the test patterns indicating concentrations of gray within a predetermined concentration range including the reference concentration, the gray of the test patterns being a mixture of cyan, magenta and yellow, the gray of the reference pattern being a mixture of cyan, magenta, yellow and black; visually comparing the test patterns with the reference pattern so as to select one of the test patterns of which concentration matches the concentration of the reference pattern; and adjusting the concentration of gray in accordance with the concentration of the selected one of test patterns.
Additionally, there is provided according to one aspect of the present invention a processor readable medium which stores a program for causing a computer to carry out the above-mentioned methods.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.